Inflammatory responses to bacterial and fungal infections are primarily mediated by neutrophils and monocytes (Medzhitov, R. & Janeway, C., Jr., 2000, N. Engl. J. Med. 343:338-44; Hoffmann, J. A., Kafatos, F. C., Janeway, C. A. & Ezekowitz, R. A., 1999, Science 284:1313-8). These cells express pattern recognition receptors (PRR) which recognize conserved molecular structures shared by groups of microorganisms (Aderem, A. & Ulevitch, R. J., 2000, Nature 406:782-7; Beutler, B., 2000, Curr. Opin. Microbiol. 3:23-8). Engagement of PRRs by microbial products activate signaling pathways which control the expression of a variety of genes. These inducible genes encode proinflammatory chemokines and cytokines and their receptors, as well as adhesion molecules and enzymes that produce low molecular weight proinflammatory mediators and reactive oxygen species. The combined action of all these products presumably leads to elimination of the infectious agents and tissue repair. However, excessive secretion of pro-inflammatory mediators, together with overexpression of their receptors, cause excessive autocrine/paracrine activation of neutrophils and monocytes, leading to tissue damage and septic shock (Bone, R. C., 1991, Ann. Intern. Med. 115:457-69; Beutler, B., Milsark, I. W. & Cerami, A. C., 1985, Science 229:869-71; Morrison, D.C. & Ryan, J. L., 1987, Annu. Rev. Med. 38:417-32; Tracey, K. J. et al., 1986, Science 234:470-74; Glauser, M. P., Zanetti, G., Baumgartner, J. D. & Cohen, J., 1991, Lancet 338:732-36). Thus, the regulation of neutrophil and monocyte activation by stimulatory receptors and their ligands is crucial to the outcome of host inflammatory responses to infections.
Neutrophil- and monocyte/macrophage-mediated inflammatory responses can be stimulated through many receptors with different structures and specificities (Rosenberg, H. F., and J. I. Gallin, 1999, Inflammation. In Fundamental Immunology, 4th Ed., W. E. Paul, ed., Lippincott-Raven, Philadelphia p. 1051). These include G protein-linked seven-transmembrane domain receptors specific for either fMLP, lipid mediators, complement factors, or chemokines, the Fc and complement receptors, the CD14 and Toll-like receptors for LPS, as well as the cytokine receptors for IFN-γ and TNF-α (Ulevitch, R. J., and P. S. Tobias, 1999, Curr. Opin. Immunol. 11:19). In addition, engagement of these receptors can up-regulate or “prime” the responsiveness of myeloid cells to other stimuli, potentiating the inflammatory response (Downey, G. P., T. Fukushima, L. Fialkow, and T. K. Waddell, 1995, Semin. Cell Biol. 6:345).
Neutrophils and macrophages express additional activating receptors, but their role in inflammation is unknown. These receptors belong either to the Ig superfamily (Ig-SF), such as Ig-like transcripts (ILT)/leukocyte Ig-like receptors (LIR)/monocyte/macrophage Ig-like receptors (MIRs), paired Ig-like receptor (PIR-As), and signal regulatory protein β1 (SIRPβ1), or to the C-type lectin superfamily, such as myeloid DAP12-associating lectin-1 (MDL-1) (Nakajima, H., J. Samaridis, L. Angman, and M. Colonna, 1999, J. Immunol. 162:5; Yamashita, Y., M. Ono, and T. Takai, 1998, J. Immunol. 161:4042; Kubagawa, H., et al., 1999, J. Exp. Med. 189:309; Dietrich, J., M. Cella, M. Seiffert, H.-J. Biihring, and M. Colonna, 2000, J. Immunol. 164:9; Bakker, A. B., E. Baker, G. R. Sutherland, J. H. Phillips, and L. L. Lanier, 1999, Proc. Natl. Acad. Sci. USA 96:9792). Typically, all of these receptors bear some homology with activating NK cell receptors (Lanier, L. L., 1998, Annu. Rev. Immunol. 16:359). In particular, they contain a short intracellular domain that lacks docking motifs for signaling mediators and a transmembrane domain with a positively charged amino acid residue. This residue allows pairing with transmembrane adapter proteins, which contain a negatively charged amino acid in the transmembrane domain and a cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). Specifically, ILT/LIR/MIR and PIRs are coupled with the δ-chain of the Fc receptors (FcRγ) (Nakajima, H., supra; Yamashita, Y., supra; Kubagawa, H., supra), whereas SIRPβ1 and MDL-1 pair with DAP12 (Dietrich, J., supra; Bakker, A. B., supra). Upon ITAM phosphorylation, these adapters recruit protein tyrosine kinases, which initiate a cascade of phosphorylation events that ultimately lead to cell activation.
DAP12-deficient mice exhibit a dramatic accumulation of dendritic cells (DCs) in muco-cutaneous epithelia, associated with an impaired hapten-specific contact sensitivity (Bakker, A. B., et al., 2000, Immunity 13:345-53; Tomasello, E., et al., 2000, Immunity 13:355-64). Furthermore, recent evidence suggests that the interaction between CCR7 (CC family chemokine receptor no. 7) and ELC (Epstein-Barr virus-induced molecule 1 ligand chemokine) triggers DC trafficking to the lymph nodes. In particular, skin DCs from CCR7−/− mice, as well as in DAP12−/− mice, are severely impaired in migrating to the draining LNs following activation (Foster, R., et al., 1999, Cell 99:23-33). However, the DAP12-associated receptor responsible for these phenotypes is yet unknown.
The recent discovery of a new DAP12-associated receptor on NK cells, called NKp44 (Cantoni, C., et al., 1999, J. Exp. Med. 189:787), suggested the possible existence of yet unknown DAP12-associated receptors also on other cells involved in innate responses.
The present inventors have identified new immunoglobulin-super-family (Ig-SF) receptors designated as TREMs (triggering receptor expressed on myeloid cells), that are involved in the regulation of a variety of cellular responses, especially immune and inflammatory responses as well as trafficking of DCs.